The invention relates generally to optical access networks, and more particularly to a system and method for synchronizing telecom clocks throughout a passive optical access network.
The explosion of the Internet and the desire to provide multiple communications and entertainment services to end users have created a need for a broadband network architecture that improves access to end users. Although the bandwidth of backbone networks has experienced a substantial growth in recent years, the bandwidth provided by access networks has remained relatively unchanged. Thus, the xe2x80x9clast milexe2x80x9d still remains a bottleneck between a high capacity LAN or Home network and the backbone network infrastructure.
Digital Subscriber Line (DSL) and Cable Modem (CM) technologies offer some improvements over more conventional last mile solutions. However, these technologies still do not provide enough bandwidth to support emerging services such as Video-On-Demand (VoD) or two-way video conferencing. In addition, not all customers can be covered by DSL and CM technologies due to distance limitations.
One broadband access network architecture that offers a solution to the xe2x80x9clast milexe2x80x9d problem is a point-to-multipoint passive optical network (PON). A point-to-multipoint PON is an optical access network architecture that facilitates broadband communications between an optical line terminal (OLT) and multiple remote optical network units (ONUs) over a purely passive optical distribution network. A point-to-multipoint PON utilizes passive fiber optic splifters and combiners to passively distribute optical signals between the OLT and the remote ONUs.
In the past, much of the PON development has been focused on ATM-based PONs. However, in recent years, there has been a growing interest in Ethernet-based PONs. This growing interest is partly due to the fact that about ninety-five percent (95%) of LANs currently use the Ethernet protocol. Therefore, Ethernet-based PONs are much more preferable than ATM-based PONs to interconnect Ethernet networks. Another contributing factor is that Ethernet is more compatible with the IP protocol, which is the protocol for the Internet.
However, unlike ATM, Ethernet was not originally designed to provide synchronization of telecom clocks to facilitate proper voice transmission through an Ethernet-based network. Therefore, in an Ethernet-based PON, synchronized telecom clocks may have to be independently extracted by the OLT and the ONUs from one or more external sources, such as central offices. Alternatively, a telecom clock may have to be distributed from a single source, such as the OLT, to the rest of the network, e.g., the ONUs, over a different transmission medium than the optical fibers that interconnect the OLT and the ONUs. However, these solutions significantly increase the overall cost of the PON components, as well as increase the complexity of the Ethernet-based PON.
In view of the above concern, there is a need for a system and method for economically and efficiently synchronizing telecom clocks throughout an Ethernet-based PON.
A system and method for synchronizing clocks related to telecommunications throughout s point-to-multipoint optical network utilizes downstream data timed using a high frequency transmission clock to distribute timing information of a central telecom-based clock to remote terminals. In an exemplary embodiment, the point-to-multipoint optical network system is an Ethernet-based passive optical network (PON) system that operates in accordance with a Gigabit Ethernet standard. The timing information of the central telecom-based clock is extracted from the downstream data at each remote terminal by recovering the high frequency transmission clock and then, deriving a reference clock, which is synchronized with the central telecom-based clock, from the recovered transmission clock. The reference clock is then used to generate one or more telecom-related clocks that are needed by the remote terminal. The system and method allows telecom-related clocks throughout the system to be synchronized in an efficient and cost-effective manner.
A method of synchronizing clocks related to telecommunications in a point-to-multipoint optical network in accordance with the present invention includes the steps of deriving a telecom-based clock at a first network terminal of the optical network from an external source, generating a data transmission clock from the telecom-based clock, transmitting data in variable-length packets from the first network terminal using the data transmission clock to embed a timing information of the telecom-based clock into the data, deriving a reference clock by extracting the timing information of the telecom-based clock from the data, and generating a remote telecom-related clock from the reference clock. The data transmission clock, the reference clock and the remote telecom-related clock are substantially synchronized with the telecom-based clock. The variable-length packets may be substantially compliant to an Ethernet-based protocol, such as a Gigabit Ethernet-based protocol.
The method may further include the step of generating a transmission-based clock using the transmission rate of the data. The transmission-based clock is substantially synchronized with the data transmission clock that defined the transmission rate. In an embodiment, two phase shifted transmission-based clocks are generated using the transmission rate.
In an exemplary embodiment, the clocks that are used by the method may be as follows: the data transmission clock may be a 125 MHz clock; the telecom-based clock and the reference clock may be 8 kHz clocks; the two phase shifted transmission-based clocks may be 62.5 MHz clocks that are phase shifted by 180 degrees to each other; and the telecom-related clock may be a 1.544 MHz clock, a 2.048 MHz clock, a 51.84 MHz clock, or any multiples thereof.
A system in accordance with the present invention includes a central access module coupled to an external telecommunications network, and a number of remote terminals optically coupled to the central access module. The central access module includes a network interface that is configured to obtain a telecom-based clock from the external telecommunications network, a transmission clock generator configured to generate a data transmission clock using the telecom-based clock, and a transmitting sub-system that transmits said data in variable-length packets at a prescribed data rate, which is defined by the data transmission clock to carry timing information of the telecom-based clock with the data. The data transmission clock is substantially synchronized with the telecom-based clock. The transmitting sub-system may be configured to transmit data in variable-length packets that are substantially compliant to an Ethernet-based protocol, such as a Gigabit Ethernet-based protocol.
Each remote terminal of the system includes a receiving sub-system that extracts the timing information of the telecom-based clock from the data and generates a reference clock, and a remote clock generator configured to generate a remote telecom-related clock from the reference clock. The remote telecom-related clock is substantially synchronized with the telecom-based clock at the central access module.
The receiving sub-system of a remote terminal may include a physical layer module that generates one or more transmission-based clock from the data transmitted from the central access module, and a frequency divider operatively coupled to the physical layer module that generates the reference clock from the transmission-based clock, which is substantially synchronized with said data transmission clock. In an embodiment, the physical layer module may be configured to generate two phase shifted transmission-based clocks.
In an exemplary embodiment, the clocks that are used by the system may be as follows: the data transmission clock may be a 125 MHz clock; the telecom-based clock and the reference clock may be 8 kHz clocks; the two phase shifted transmission-based clocks may be 62.5 MHz clocks that are phase shifted by 180 degrees to each other; and the telecom-related clock may be a 1.544 MHz clock, a 2.048 MHz clock, a 51.84 MHz clock, or any multiples thereof.